In U.S. patent application Ser. No. 13/713,744 (now issued U.S. Pat. No. 8,962,377 B2), imaging array devices with metal-oxide thin-film-transistor (MOTFT) pixel readout circuits over PIN photodetector arrays were disclosed. Similar device architecture and process flow can also be used for making display devices in which each pixel comprises a MOTFT pixel driver circuit constructed with a MOTFT over a light emitting display element made in a PIN stack.
Recently, the emission efficiency of GaN LEDs has been improved rapidly to over 150 lm/W and it has become an ideal light source for projector displays due to its long operation lifetime compared with the gas discharge lamp. However, using LEDs to replace the gas discharge lamp is limited by the size of the LED. It is hard to make a large uniform LED source that can work with a flat liquid crystal display (LCD) or digital micro-mirror display (DMD) engine. The major issues are current crowding and the loss of homogeneity as the LED gets larger. The present invention uses an array of LEDs to overcome these issues. Because of the patterning into individual small LEDs (<100 microns), there are no issues of current crowding and inhomogeneity in driving the LEDs. The LED array consists of MxN LEDs where each LED can be driven independently to create the image. Each LED is very small and efficient without current crowding. To generate 5000 lm in a projector, assuming efficiency of 100 lm/W, one needs 50 W and 16 A of current (3V for GaN based PIN emission junction). Using current density of 10 A/cm2, only an effective emission area of 1.6 cm2 is needed. So the emitter array can be made in a very small area. Assuming a moderate fill factor (emitting area within each display pixel) of 30%, less than 5 cm2 of GaN area is needed. The 10 A/cm2 driving current is well below what the GaN LED is capable of (at 100 A/cm2). So the capability to increase the luminance by another factor of 10; i.e., 50000 lm, is possible. When such active matrix LED display (AMLED) is used in a projector for a Movie Theater or for an outdoor display at brightness of 500 cd/m2, over 100 square meter screen can be covered. Such AMLED is, thus, capable of providing an extremely high power projector. A key challenge for such a high power density projector is to dissipate the heat. The heat removal through thermal conduction alone is difficult because of the other requirements: the heat spreading material in such a device has to be transparent. The best transparent heat spreader is the SiC or GaN substrate that are used to grow the GaN LED. Sapphire is also transparent, but its heat spreading capability (23 W/m-K) is much smaller than SiC (120 W/m-K) or GaN (130 W/m-K). GaN has the additional benefit of lattice matching and enables better quality crystal growth. GaN can also be grown on Si, but it is not transparent and the substrate has to be removed in order to optimize its optoelectronic performance.
Another challenge is to provide an active matrix to drive the large array of M×N individual LEDs. There are two solutions to this problem. One solution is to make m×n connections to Si driver chips by solder bumps. Besides the solder bump yield is challenging to make the millions of connections, there is another serious problem in using a Si chip right on top of the GaN LED array. The high temperature of the GaN LED junction may cause the Si IC to malfunction. The leakage current of a-Si field effect transistor (FET) increases rapidly with temperature because of its lower bandgap (1.1 eV). To make the active matrix work, for such small pixel size, the FET leakage current has to be very small due to the limited size of storage capacitance. FETs based on silicon wafer are thus difficult for such high power density, high temperature applications.
The other solution is to make an active matrix directly on GaN LED wafers using TFT technology of higher bandgap to solve the leakage current problem as disclosed in detail in this invention. The leakage current of a high bandgap MOTFT is not strongly dependent on temperature and can be kept small at temperature even above 100° C. (Gang Yu, IMID 2014). Also a high bandgap MOTFT can be made directly on top of a GaN LED and there is no need to use the low yield solder bump to make the millions of connection.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide high pixel density, active matrix LED arrays for high power projector display.
It is another object of the present invention to provide an AMLED display with a MOTFT pixel driver circuit on top of a PIN LED array without a solder bonding layer.
It is another object of the present invention to provide an AMLED display with improved thermal conduction, improved current density and improved output optical power.
It is another object of the present invention to provide a new and improved process for fabricating an active matrix light emitting/sensing array with a metal-oxide thin film transistor (MOTFT) array.
It is another object of the present invention to provide a new and improved process for fabricating AMLED arrays using a fewer number of process steps.
It is another object of the present invention to provide a new and improved process for fabricating AMLED arrays with high pixel counts and high pixel density for large screen projectors used in indoor and outdoor applications.
It is another object of the present invention to provide a new and improved process for fabricating an AMLED array without a solder bond layer and with improved operation stability.
It is another object of the present invention to provide a new and improved process for fabricating AMLED displays with more pixels and with smaller pixel pitch.